This invention is concerned with interferometers. In particular the invention is directed to a Mach-Zehnder interferometer wherein the light source is split into two beams which propagate via two separate paths. The beams are then recombined at the detector. If the optical path length differs between the two paths, then an interference effect is observed at the detector as a variation in detected light intensity.
The best performance for high resolution interferometer applications is obtained where the light phases are constrained to operate near the so-called quadrature point. This is illustrated in FIG. 1. In this region, the relationship between the path length difference and the optical intensity is nearly linear.
The optical path length may be extracted from the time variation of the light intensity as seen by the detector. Displacement demodulation schemes typically utilize either fringe counting or phase demodulation. The fringe counting technique is typically able to resolve displacements limited to a relatively large fraction of one interference fringe (.lambda.). In addition fringe counting techniques are not generally suited to high frequency (MHz) applications. In phase demodulation, the interference fringe is maintained at the quadrature position by some feedback mechanism. Small changes about the quadrature point can then be detected.
The resolution capabilities of this technique are intrinsically high and limited only by the noise level of the system. Additionally, the frequency response characteristics of such a system can be very good, easily demodulating phase variations with frequency content up to 50 MHz.
A problem with this technique occurs when the interference condition is driven to or beyond an interference extremum. At the extremum, the sensitivity of the technique is identically zero. If the interference condition is driven beyond an extremum, then ambiguity occurs as to the change in phase angle. One can only know the phase angle to within an additive constant n.lambda./2, where n may take on integer values.
As a result, the fringe counting technique offers good range characteristics with limited resolution and bandwidth, while the phase demodulation method offers good resolution and bandwidth characteristics with a very limited displacement range. For certain applications it is desirable to obtain the best characteristics of each demodulation scheme combined in a single method wherein one obtains good resolution and bandwidth, as well as extended range.
The present invention is directed to minimizing the difficulties in the prior systems.